1. Field of the Invention
This invention relates to a method of fabricating a semiconductor nonvolatile storage device (nonvolatile memory), more particularly a method of fabricating a semiconductor nonvolatile storage device of metal--oxide film--nitride film--oxide film--semiconductor (MONOS) structure.
2. Description of the Related Art
Semiconductor nonvolatile storage devices that enable electrical rewriting of data are referred to generally as EEPROMs. Various types are known. The main ones are the MONOS memory, the MNOS memory and the floating gate memory.
The MONOS memory is a semiconductor nonvolatile storage device having a metal--oxide film--nitride film--oxide film--semiconductor (MONOS) structure. It has attracted attention owing to its high reliability and its ability to permit any number of data rewrites.
The structure of the semiconductor nonvolatile storage device having the MONOS structure and the conventional method of fabricating it will be explained with reference to FIGS. 23 to 25.
To fabricate a semiconductor nonvolatile storage device of the MONOS structure, an element isolation region 15 composed of a silicon nitride film is formed on a semiconductor substrate 14 of P-conductivity-type silicon, as shown in FIG. 23.
A silicon oxide film for providing a tunnel oxide film 1 is then formed over the whole surface of the semiconductor substrate 14 by thermal oxidation.
Next, a silicon nitride film is formed over the whole surface of the semiconductor substrate 14 by the chemical vapor deposition (CVD) process. This silicon nitride film is subjected to thermal oxidation to form a silicon oxide film for providing a top oxide film 3.
After this, a polycrystalline silicon film for providing a memory gate electrode 4 is formed over the whole surface of the semiconductor substrate 14 by the chemical vapor deposition process.
A patterned photo resist (not shown) is then formed. The patterned photo resist is used as an etching mask to sequentially etch the polycrystalline silicon film, the silicon oxide film, the silicon nitride film and the silicon oxide film to pattern the memory gate electrode 4 and a memory gate insulating film 20 composed of the top oxide film 3, a silicon nitride film 2 and the tunnel oxide film 1, as shown in FIG. 23. The photo resist used as the etching mask is thereafter removed.
The tunnel oxide film 1 is an insulating film for injecting or emitting electrons or holes. The silicon nitride film 2 is an insulating film whose function is to trap electrons or holes injected through the tunnel oxide film 1. The function of the top oxide film 3 is to block injection of electrons or holes from the memory gate electrode 4 when the semiconductor nonvolatile storage device effects write or erase operation.
Next, high-concentration diffused regions 5, 5 are formed by implanting phosphorus ions into the element region of the semiconductor substrate 14 at prescribed portions thereof self-aligned with opposite sides of the memory gate electrode 4 and the device isolation region 15.
As shown in FIG. 24, an interlayer insulator 6 composed of a silicon oxide film is then formed over the whole surface of the semiconductor substrate 14 by the chemical vapor deposition (CVD) process.
A patterned photo resist (not shown) is then formed. The patterned photo resist is used as an etching mask to form contact holes 7 in the interlayer insulator 6 by etching. The photo resist used as the etching mask is thereafter removed.
Next, an interconnection material composed of aluminum is formed over the whole surface of the semiconductor substrate 14, including the insides of the contact holes 7, by sputtering.
A patterned photo resist (not shown) is then formed and the interconnection material is plasma-etched using the patterned photo resist as a mask to form a gate interconnection electrode 8G, a source interconnection electrode 8S and a drain interconnection electrode 8D that make contact through the contact holes 7 with the exposed portions of the memory gate electrode 4 and the heavily doped diffusion regions 5, 5.
Following this, the photo resist used as the etching mask is removed and the interconnection electrodes are annealed in a hydrogen atmosphere to lower the contact resistance between the source interconnecting electrodes 8S, 8D and the heavily doped diffusion regions 5.
Next, as shown in FIG. 25, the whole surface of the semiconductor substrate 14 is formed with a passivating film 10 composed of a silicon nitride film by the plasma chemical vapor deposition (CVD) process.
Openings 10a are thereafter phot-etched in the passivating film 10 to expose the interconnection electrodes 8G, 8S and 8D at input/output terminal portions 11 for enabling connection with an external device.
Although FIG. 25 shows only the input/output terminal portion 11 of the source interconnection electrode 8S and the associated opening 10a, an input/output terminal portion and an associated opening are also formed at a different sectional portion from that shown in FIG. 25 for each of the gate interconnection electrode 8G and the drain interconnection electrode 8D.
This step of forming the openings in the passivating film 10 completes the fabrication of the semiconductor nonvolatile storage device.
When this conventional method of fabricating a semiconductor nonvolatile storage device of MONOS structure is adopted, the semiconductor nonvolatile storage device is damaged in the step of etching the interconnection material, the step of forming the passivating film and/or the step of etching the passivating film.
This damage is caused by the step of plasma-etching the interconnection material, the plasma chemical vapor deposition process during passivating film formation, and the step of plasma-etching the passivating film.
Specifically the plasma separates the etching gas or the material for forming the passivating film into positive charges and negative charges and the positive charges or negative charges charge the memory gate electrode 4 during etching or passivating film formation. When the memory gate electrode 4 is positively charged, electrons from the semiconductor substrate 14 are trapped in the memory gate insulating film 20, and when it is negatively charge, holes from the semiconductor substrate 14 are trapped in the memory gate insulating film 20.
Such charging of the semiconductor nonvolatile storage device is equivalent to applying a write or erase voltage to the memory gate electrode 4, and has the same effect. Specifically, the electrons or holes trapped in the memory gate insulating film 20 change the threshold voltage of the semiconductor nonvolatile storage device.
This change in threshold voltage changes the threshold voltage during write and erase. Since this may prevent the semiconductor nonvolatile storage device from effecting normal write and erase, it is a cause of defective operation.